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Displacement and Position Sensors (Part 1)

Written by George Novacek
Measurement Techniques Object tracking is a required feature in many modern control systems. In this article, George examines measurement techniques used generate electrical output with respect to distance or position. Without displacement and position sensors, robotics and position control systems couldn’t exist. In this article, I’ll cover measurement techniques generating electrical output with respect to distance or position. The terminology encountered in the literature on this subject is not very consistent, so to avoid confusion, I use the term displacement sensors to refer to devices that continuously track an object’s movement. Position sensors are those that indicate when a target object has reached its destination. Displacement sensors can be divided into three major categories: linear, rotary, and contactless. Once again, an ambiguity exists in literature as to what constitutes a contactless sensor. LVDT & RVDT Some refer to Linear Variable Differential Transformers (LVDT) as contactless, as opposed to, for instance, potentiometers, whose wiper and thus circuit is attached to the measured object. For clarity, I shall call sensors contactless if and only if none of their parts are mechanically coupled with the measured object, such as in radar or ultrasonic ranging. LVDTs have been around for about a century. With their unlimited resolution while working in the harshest environments, including space, they have been the undeclared kings of displacement sensors. Figure 1 depicts an LVDT. The primary coil is excited by an AC signal. The core of the LVDT is attached to the object whose movement is to be tracked. As the core moves, the inductive coupling between the primary and the secondary windings changes, which in turn changes the output voltages of the secondary windings. Figure 1Figure 2 Figure 1 This is a simplified cross-section of an LVDT. Coils are often tapered or otherwise f
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Displacement and Position Sensors (Part 1)

by George Novacek time to read: 6 min